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New research shows the long-term impact of electrical discharges on composites – and the measures that can be taken by industry
In his recently completed PhD thesis, Søren Find Madsen of Ørsted-DTU, Technical University of Denmark, shows how industry can obtain valuable information about hidden defects in composite materials using a simple standardised test method.
“There is a clear correlation between the surface and interior of composite materials. Therefore, it is in practice sufficient to measure the tracking resistance of a composite and calculate its breakdown field strength from that. This saves on expensive testing in high voltage laboratories,” says Søren Find Madsen.
Søren Find Madsen’s research shows how the breakdown field strength of a composite material can be calculated from the material’s tracking index by using a relative simple formula.
The formula looks like this:
Breakdown field strength Eb = TI x C1 + C2/d
Where:
TI = tracking index
d = the material thickness
C1 and C2 are empirical constants found during the tests
A composite with a low tracking index will typically also have a relatively low breakdown voltage. The low tracking index is normally a result of substandard production quality, which is also reflected further into the material.
“A rough or uneven surface, for example with loose fibres, produces a low tracking index, and in these products defects are normally also found inside the material. Critical defects where breakdown voltage is concerned include air pockets, cracks and impurities. The problems typically occur at interfaces between different materials in the composite, for example between resin and glass,” says Søren Find Madsen.
Destruction normally starts inside these defects because the electrical field is typically larger than in the surrounding resin, and because the breakdown voltage for, say, air-filled cavities is considerably smaller than for the pure composite. For example, in a large electrical field, small discharges will occur in a material cavity. Such a defect in a finished product cannot be detected immediately, but can in time cause a collapse of the structure. Temperatures of 20,000 – 30,000 oC can occur in such a cavity and continuously erode the composite from within.
“Such defects are not usually visible to the naked eye, but microscopic examination reveals the small internal flaws that cause long-term effects and finally dielectric breakdown. It is these flaws that we can very easily identify with the new calculation method,” says Søren Find Madsen.
Søren Find Madsen’s findings have been published in his final PhD thesis and in several research reports authored in partnership with other researchers at the Technical University of Denmark. A bibliography can be found on Søren Find Madsen’s website: www.highvoltage.dk.
Two of the articles covering studies of tracking characteristics and breakdown strength can be read here:
http://www.fiberline.dk/gb/articles/art5184.asp
http://www.fiberline.dk/gb/articles/art5143.asp
Read also a previous article on the subject: http://www.fiberline.dk/gb/newsroom/press5077.asp
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